A PHENGITE + DUMORTIERITE – BEARING VEIN IN THE NORTHERN GOOCHLAND TERRANE, VA: EVIDENCE FOR RAPID TERRANE EXHUMATION

The Goochland Terrane of the eastern Virginia Piedmont preserves evidence of an early, moderate-pressure sillimanite+Kfeldspar-producing granulite metamorphic event that was overprinted by a higher-pressure kyanite-producing event (Farrar, 1984). This second event increased in pressure and temperature to the northeast, reaching kyanite-Kfeldspar granulite conditions in strongly deformed, migmatitic metapelites along the North Anna north of Richmond, with P-T conditions calculated at P=10.2 kbar, T= 730-740 ℃. (Owens et al. 2012).

A single, 2 cm thick, undeformed, quartzo-feldspathic vein which cuts the kyanite-Kfeldspar migmatitic gneiss at a high angle to foliation, defines a part of this terrane’s P-T-t path not previously recognized. The vein is coarse-grained, with the assemblage quartz-plagioclase-phengite +/- dumortierite. The coarse phengite grains are, in part, pseudomorphically replaced by the assemblage Kfeldspar-kyanite+/-biotite+/-dumortierite. How can the undeformed nature of the vein, both in shape and texture, be explained as having formed at such high P-T conditions? A texturally and mineralogically nearly identical set of veins near Wilson Lake , Labrador, were explained as having formed in the early stages of rapid, nearly isothermal, uplift of a terrane from great depth by slip on bounding mylonite zones, causing two stages of decompression melting which produced the veins along planar fractures and melted phengite to produce pseudomorphs (Korhonen and Stout, 2005). Applied here, this vein formation defines a high pressure, early stage of rapid terrane exhumation (from about 11 to 9 kbar P, at about 800℃), perhaps during orogenic collapse of the Appalachians in the late Alleghanian (?) with strain partitioned to the adjacent Hylas mylonite zone, and no deformation of the vein.